1. Field of the Invention
This invention relates to an apparatus and process for the activation and reactivation of carbon by electrical resistance heating in the presence of steam. It particularly relates to an improved apparatus in which both the electric current and the steam are more uniformly distributed through the carbon, and in which the paths of both the electric current and the steam flow can be selected so as to achieve optimal flexibility in the operation of the apparatus.
2. Description of the Related Art
The use of electrical resistance heating for the activation and reactivation of carbon in the presence of steam has been described in U.S. Pat. Nos. 5,089,457, and 5,173,921. In the processes described therein, the carbon to be activated or reactivated descends by gravity through a tubular reactor, while steam is fed into the bottom of the reactor and moves upward in opposition to the descending carbon. The steam can undergo any or all of a number of possible reactions:
1. In carbon activation, the steam and carbon are converted to carbon monoxide and hydrogen, according to the equation EQU H.sub.2 O.sub.gas +C.sub.solid .fwdarw.CO.sub.gas +H.sub.2.sbsb.gas PA1 2. Some of the steam can react further with carbon monoxide in the "water gas shift reaction." EQU H.sub.2 O.sub.gas +CO.sub.gas .fwdarw.CO.sub.2.sbsb.gas +H.sub.2.sbsb.gas PA1 3. The steam also serves to desorb other matter that is already adsorbed on spent activated carbons. PA1 4. The steam can react with some adsorbates on spent carbons, as for example in the hydrolysis of adsorbed fatty esters.
Other gases can also be used to activate carbon. A useful example is carbon dioxide, which reacts with carbon according to the following equation: EQU CO.sub.2.sbsb.gas +C.sub.solid .fwdarw.2CO.sub.gas
In carbon activation, reactions 1 and 2 consume steam, so that the activation process is slowed down in the upper portions of the tubular reactor, or else an excess of steam must be provided in the initial injection to make up for such consumption. Such excess seriously compromises the controls needed to establish proper reaction temperatures for activation. In carbon reactivation, the desorbed vapors from reactions 3 and 4 may recondense in the upper portions of the reactor, thereby offering increased resistance to gas flow ("back pressure") and unfavorably affecting the temperature and current distributions in the reactor. Also, if all the steam needed for its various functions is introduced at one point at the bottom of the reactor, then its rate of introduction must be limited so as not to reach a linear speed that would impose too great a pressure on the carbon or require too high a reactor temperature. These limitations necessarily slow down the activation or reactivation processes. Another disadvantage of rapid steam introduction is the possibility of lifting or incipient fluidizing of the carbon bed, resulting in arcing of the electric current between carbon particles. Another disadvantage of rapid steam introduction into carbon particles that are heated by electrical resistance is the tendency for separation of the carbon particles at the injection points with consequent electrical arcing and burning of the steam injector tips. Still another major disadvantage is the fact that excessive steam flow beyond what is needed in a particular section of the reactor may have an adverse effect on the temperature profile in the reactor. In summary, the introduction at one point in the reactor of the total steam flow needed for all requirements in the reactor severely limits the options for operating controls in the activation or reactivation processes by electrical resistance heating.
It is accordingly an object of this invention to provide an improved apparatus for the activation of carbon feedstocks or the reactivation of spent activated carbon by electrical resistance heating in the presence of an activating gas.
It is another object of this invention to provide such improvement when the activating gas is steam or carbon dioxide.
It is another object of this invention to limit the introduction of steam to any section of the reactor to the amount of steam needed for the processes occurring in that section, and to allow the gaseous reaction products or the desorbed gases and vapors to escape from the section of the reactor where they are produced.
It is another object of this invention to inject the steam into the carbon by a means that will distribute the steam uniformly through the carbon particles and prevent electrical arcing and consequent burning at the steam injector points.
It is yet another object of this invention to provide that the electric current is distributed uniformly through the carbon being activated or reactivated in each section of the reactor.